1. Field of the Invention
This invention relates to a variable valve lift device for varying the opening and closing timing or amount of lift of intake and exhaust valves in order to regulate the amount of suction and exhaust at low and high rotational speed of an engine.
2. Discussion of the Related Art
In the engine of an automobile or the like, the amount of suction and exhaust required is small immediately after the engine is started and when the engine is running at low rpm or under a small load. The amount of suction and exhaust required increases when the engine is running at high rpm or under a high load. It has therefore been contemplated to obtain appropriate engine output and reduce the amount of fuel consumption by controlling the opening and closing amount of the intake and exhaust valves as well as the operation timing of the valves.
A variable valve lift device for achieving this is described in the specification of Japanese Patent KOKAI Publication No. 6(1994)-17630 entitled "Variable Valve Timing and Lift Mechanism". In the disclosed variable valve lift device, first through third cylinder holes are arranged in a direction which perpendicularly intersects the axial direction of outer and inner lifters. The first through third cylinder holes are so arranged as to connect coaxially when respective ones of the outer and inner lifters come into contact with the base circles of respective ones of a high-speed cam (outer cam) and a low-speed cam (inner cam) so that the top surfaces of the outer and inner lifters lie in the same plane. In addition, a rotation preventing mechanism for preventing relative rotation between the outer and inner lifters is so arranged that the first through third cylinder holes will lie on the same axis. First through third pins are slidably fitted into respective ones of the coaxially connected cylinder holes, and a spring which urges the third pin toward the side of the second pin at all times and a stopper are disposed within the third pin. A changeover valve reduces hydraulic pressure to the first cylinder hole when the engine is running at low rpm and increases hydraulic pressure to the first cylinder hole at high rpm.
However, according to the inventor's investigation it has turned out that a problem arises in the above-described arrangement in which the top surfaces of the upper and lower lifters reside in the same plane when the upper and lower lifters are in abutting contact with the base circles of the outer and inner cams, respectively. Specifically, at low rpm, namely when the first through third pins make it possible for the outer lifter and inner lifter to move up and down relative to each other, the inner cam contacts the peripheral portion of the top surface of the rising inner lifter, thereby causing wear or deformation of the cam or lifter.
Further, in order to fit the cylindrically shaped first through third pins, the outer and inner lifters must each be increased in thickness in the axial direction to provide sufficient strength. In addition, the cylindrically shaped pins have a pin diameter that is greater than the thickness of a plate because of strength considerations, and the result in a device having a large overall bulk.
Further, the outer periphery of the outer lifter is provided with holes for receiving the fitted pins. Owing to constant sliding within a guide hole, this causes fatigue fracture at this portion of the outer lifter.
Furthermore, since relative rotation between the outer and inner lifters is prevented by the rotation preventing mechanism, the inner and outer cams, for which the driving forces are different, sustain wear at the surfaces of contact between the lifters. In addition, a force which attempts to rotate the outer and inner lifters is produced and causes deformation of the pins or rotation preventing mechanism.